This invention relates to a new system for improved quality and process control in the wood products industry, and in particular, to a system for determining resin distribution in cellulosic material by using a novel labelled resin for detection by x-ray spectrometry.
Cellulosic composite products are generally manufactured by organizing cellulosic material, such as particles of wood, straw, bamboo, hemp or the like into a mat of material after coating by a bonding agent, and exposing the resulting mat to pressure and heat to create a finished product such as a board or panel. In the case of wood, the particles can include chips, flakes, fibres or strands. The bonding agent plays a crucial role in determining the strength of the finished product. In the medium density fibreboard and particleboard industry, urea-formaldehyde resins are preferably used. Other resins include melamine-urea-formaldehyde, melamine formaldehyde, and phenol-formaldehyde resins.
The amount of resin added and the overall distribution of the resin are key factors in determining the strength of the finished product. Presently, it is standard practice to blend a pre-determined amount of resin with the cellulosic material and test samples of the resulting product to determine strength properties. Depending on strength test results, the amount of resin added is adjusted downwardly to lower the strength of the product while reducing cost or adjusted upwardly to increase strength and costs. Resin is among the costliest components of a composite cellulosic product and being able to reduce the amount of resin while ensuring that the final product meets quality control and assurance guidelines is a sought after goal in the industry.
Using less resin requires that the resin be distributed as efficiently as possible in the finished product. Reliable measurement of resin distribution has been a long standing problem for the industry particularly with respect to composite wood products which represent the majority of products produced. The problem has been particularly difficult for the medium density fibreboard and particleboard industry which use urea-formaldehyde resins. No reliable and effective non-destructive test method for the detection and measurement of the resin is currently known. Urea-formaldehyde resin is nearly colourless when viewed in white light. Phenol-formaldehyde resin has a distinctive red-brown colour, but when it appears in a thin layer, it is also difficult to detect against a brown coloured wood background. Visual inspection systems are therefore not appropriate for determining resin distribution.
Work has been conducted to discover reliable methods for determining resin distribution. For example, Kasper and Chow (1980) in their paper entitled Determination of Resin Distribution in Flakeboard Using X-Ray Spectrometry, Forest Products Journal 30(7):37-40, examined phenol-formaldehyde resin distribution in wood flakes using bromide as a label and X-ray spectrometry as a detection tool. Using bromide as a label for resins has some fundamental limitations for detection and measurement of resin distribution. In particular, bromide is negatively charged and is therefore, not strongly attracted to the resin molecules which have many highly electronegative functional groups. Therefore, bromide will tend not to stay with the resin molecules throughout the board manufacturing process. In addition, bromide is highly water soluble and will tend to migrate through the wood flakes along with water. The factors impose serious limitations on bromide as a resin label.
Johansson et al. (1991) in a paper entitled A Method for the Analysis of the Glue Efficiency in Particleboards Trxc3xa4tek, Rapport I 9112076, Stockholm describe how they developed a method for the analysis of resin efficiency in particleboard using copper sulphate and rubeanic acid. The test method is destructive in that it turns the resinated material black to indicate the presence of resin. Such a method has great limitations for developing into a practical test method for both on-line and off-line measurement of resin distribution in the composite wood product industry.
The most current work to develop an effective and reliable method of detecting urea-formaldehyde resin distribution has been undertaken by Kamke. In a paper presented at the Wood Adhesives 2000 meeting, Kamke discussed using fluorescent dyes to track resin distribution, however, the results of initial test were inconclusive.
Good resin distribution is key to the manufacture of high quality cellulosic products, and, in particular, composite wood products at reasonable cost. There is a need for a reliable method of monitoring resin distribution in order that the distribution can be optimized during the resin blending and application process.
The present invention addresses the problem of measuring resin distribution by providing a non-destructive method of determining resin content and distribution. Accordingly, the present invention provides a method for measuring bonding agent content and distribution in a cellulosic material mixed with the bonding agent comprising the steps of:
using a bonding agent that includes electronegative functional groups and an X-ray active cationic label bonded with the functional groups;
exposing the cellulosic material to X-rays to generate a characteristic fluorescence signal from the label; and
measuring the X-ray fluorescence of the label to determine the amount and distribution of the label in the cellulosic material and thereby the amount and distribution of the bonding agent in the cellulosic material.
In a further aspect, the present invention provides a bonding agent composition useful for determining the distribution of the bonding agent in a cellulosic material mixed with the bonding agent comprising:
a bonding agent containing electronegative functional groups; and
a label compound that includes positively charged ions to emit characteristic x-ray fluorescence when exposed to X-rays, the bonding agent and the label compound being homogeneously mixed.
Preferably, the bonding agent will be a water based resin that contains electronegative functional groups and the label compound will contain an X-ray active metallic ion. To trace bonding agent distribution successfully, it is essential that the label stays with the bonding agent at all times during and after the manufacturing process. The metallic ions are positively charged and form strong bonds with the highly electronegative functional groups of the resin molecules to move with the resin as the resin is distributed during the manufacturing process. Subsequently, the metallic ions can be located and measured by X-ray fluorescence and converted to resin content by a standard calibration curve.